Periodic Reporting for period 1 - STORM (Synthesis of Transition metal dichalcogenides Optimized for MRAMs)
Reporting period: 2022-11-01 to 2024-10-31
The overarching objective of STORM was to find the most effective way to synthesize monolayers of the transition metal dichalcogenides (TMDs) TaTe2 and WTe2, which exhibit a 1T’ polymorphic phase in their bulk form and to then chemically functionalize them for their integration in magnetic RAMS (MRAMs).
The specific objectives of the project were:
Obj1: Replacing part of the Te atoms in the atomic lattice with other atomic species, reported as a viable route to gradually tune the electronic properties of the TMDs;
Obj2: Realizing Janus TMDs, with the formula MXY (M= Mo, W, etc; X= S, Se, Te, Y=/X).J-TMDs can be thought of as an extreme case of substitutionally doped TMDs, with the transition metal sandwiched between different chalcogens;
Obj3: Synthesizing TMD/ferromagnet heterostructures on the pathway for an industrially relevant prototype.
While the synthesis of the proposed materials was successful, the properties that these monolayers displayed were not the ones theoretically expected, meaning that they could not be functionalized for the desired purposes. We therefore devoted our attention to MoSxSe1-x (x in the 0÷1 interval) crystals and characterized their electronic properties as a function of their chemical composition (S:Se ratio).
As the project duration was shortened by 6 months over a total of 2 years, the last objective was not attempted (it was in fact scheduled for the last semester of the action).
The specific objectives of the project were:
Obj1: Replacing part of the Te atoms in the atomic lattice with other atomic species, reported as a viable route to gradually tune the electronic properties of the TMDs;
Obj2: Realizing Janus TMDs, with the formula MXY (M= Mo, W, etc; X= S, Se, Te, Y=/X).J-TMDs can be thought of as an extreme case of substitutionally doped TMDs, with the transition metal sandwiched between different chalcogens;
Obj3: Synthesizing TMD/ferromagnet heterostructures on the pathway for an industrially relevant prototype.
While the synthesis of the proposed materials was successful, the properties that these monolayers displayed were not the ones theoretically expected, meaning that they could not be functionalized for the desired purposes. We therefore devoted our attention to MoSxSe1-x (x in the 0÷1 interval) crystals and characterized their electronic properties as a function of their chemical composition (S:Se ratio).
As the project duration was shortened by 6 months over a total of 2 years, the last objective was not attempted (it was in fact scheduled for the last semester of the action).
In the first WP of the project we set out to grow and characterize single atomic layers of WTe2 and TaTe2, chosen because they exhibit a monoclinic 1T’ crystal structure in their bulk form . This should have enabled them to generate out-of-plane spin orbit torques (i.e. a longitudinal current through the atomic layer results in an out-of-plane torque, capable of flipping the magnetization of an adjacent layer). Both materials were successfully synthesized, however:
- WTe2 does not form, despite the optimization of the growth process, continuous film of a single atomic layer before the seeding of a second layer is observed; the bilayer has a different symmetry to the monolayer and cannot generate out of plane torques
- TaTe2 in its monolayer forms grows in different crystallographic phases (1H and 1T), which cannot generate out of plane torques
To maximize the impact and outcome of this MSCA action, we therefore shifted our attention to the study of MoSxSe1-x alloys with different stoichiometric ratios (namely: MoS2, MoS1.5Se0.5 MoSSe, MoS0.85Se0.15 MoS2). We carried out a characterization of these crystals with a combination of microscopic and spectroscopic techniques as a function of sample thickness and chemical composition (the first in its kind for completeness of results). We observed that these samples, which are air-stable and do not exhibit phase change with temperature, have electronic and morphological properties which change smoothly as a function of chemical composition and thickness. In particular, our preliminary results indicate that the electronic bandgap in MoSxSe1-x, which is the range in which it can absorb energy i.e. from the electromagnetic spectrum, can be seamlessly tuned in the 1.6-1.9 eV range.
- WTe2 does not form, despite the optimization of the growth process, continuous film of a single atomic layer before the seeding of a second layer is observed; the bilayer has a different symmetry to the monolayer and cannot generate out of plane torques
- TaTe2 in its monolayer forms grows in different crystallographic phases (1H and 1T), which cannot generate out of plane torques
To maximize the impact and outcome of this MSCA action, we therefore shifted our attention to the study of MoSxSe1-x alloys with different stoichiometric ratios (namely: MoS2, MoS1.5Se0.5 MoSSe, MoS0.85Se0.15 MoS2). We carried out a characterization of these crystals with a combination of microscopic and spectroscopic techniques as a function of sample thickness and chemical composition (the first in its kind for completeness of results). We observed that these samples, which are air-stable and do not exhibit phase change with temperature, have electronic and morphological properties which change smoothly as a function of chemical composition and thickness. In particular, our preliminary results indicate that the electronic bandgap in MoSxSe1-x, which is the range in which it can absorb energy i.e. from the electromagnetic spectrum, can be seamlessly tuned in the 1.6-1.9 eV range.
1. We developed, in WP1, robust procedures for the synthesis of the proposed TMDs. The results collected on tuning the growth parameters of TaTe2 have been collated in an open access manuscript (published during the fellowship, but submitted prior to its start). While processing the data collected for WTe2 we became aware of the publication of a similar work by another team of researchers, so publication of this data has been put on hold.
2. The results obtained for MoSxSe1-x can contribute to the development of ultra-thin film and air stable components for the realization of photovoltaic cells. 2D materials are currently heavily explored both as light-harvesting materials and as other components of solar cells, and we demonstrated the possibility of obtaining, by controlling thickness and chemical compositions, films with tailored electronic properties which are suitable for the realization of tandem solar cells.
2. The results obtained for MoSxSe1-x can contribute to the development of ultra-thin film and air stable components for the realization of photovoltaic cells. 2D materials are currently heavily explored both as light-harvesting materials and as other components of solar cells, and we demonstrated the possibility of obtaining, by controlling thickness and chemical compositions, films with tailored electronic properties which are suitable for the realization of tandem solar cells.